1. Field of the Invention
The present invention relates to a sealing apparatus using a ferrofluid (a magnetic fluid). Particularly, the present invention relates to a sealing apparatus for a spindle motor requiring a hermetic seal between two environments where one environment needs to be free from contamination arising from the environment on the other side of the hermetic seal. More particularly, the present invention relates to a sealing apparatus for a spindle motor used in driving a magnetic recording medium in the information industry and other fields. Yet more particularly, the present invention relates to a sealing apparatus used in the information industry and other fields for sealing magnetic recording medium and a drive mechanism with a ferrofluid so as to prevent the magnetic recording medium from being adversely affected by dust or other contaminants in the drive mechanism.
2. Description of the Prior Art
Conventional centrifugal seals employ the principle of centrifugal effect at high speeds to retain a fluid for a sealing purpose. Such conventional centrifugal seals typically have a shaft supported by a cylindrical housing by way of a ball bearing and both are structured to become relatively rotatable at will. These conventional seals use a lubricating liquid, such as oil, to effect the seal when the rotating member has reached a high speed. Such conventional centrifugal seals of this type function satisfactorily in the high-speed mode, but, at lower shaft speeds, the seal fluid is not retained in place. One early, well-known form of a non-contacting seal for use at zero and low speeds is the magnetic ferrofluid seal. However, these early magnetic ferrofluid seals tend to break down and leak at high rotational speeds. An example of an early form of a ferrofluid seal is shown in FIG. 1. A shaft member is rotatably supported by ball bearings in a housing. A ring-like magnet is interposed between the media side (upper side of the drawing) and the side of the ball bearing. On both sides of the magnet, pole pieces having an inner diameter slightly larger than the diameter of the shaft member are installed forming gaps between the pole pieces and the shaft member. The gaps are filled with a ferrofluid. In such an arrangement, the ferrofluid is retained in the gaps by magnetic force of the ring-like magnet forming a hermetic seal at the gaps on both sides.
However, in this type of sealing configuration, the shaft member needs to be made of a magnetic material and needs to be polished to a mirror-like finish to prevent the ferrofluid from bleeding, i.e. the ferrofluid does not spread along the surface of the shaft member. In addition, when the shaft member starts rotating relative to the housing, centrifugal force is exerted on the ferrofluid. When the centrifugal force exceeds an allowable range, the hermetic seal is broken. Further, a cover is normally mounted to prevent the hand from touching the injected ferrofluid at the time of handling thereof and the apparatus is enlarged by an amount of the mounting space required by the cover.
Seals were later developed to overcome the deficiencies of the magnetic seal at high-speed operation and the centrifugal seal at low speed operation. There has been developed an apparatus in which a shaft-member shielding plate is installed between the pole pieces from the opposing side, as illustrated in Prior Art FIG. 2. The gap between the housing and the shaft member is made to detour in a U-shaped passageway. During high-speed operation, a centrifugal sealing mode is established. When the seal is rotating, the ferrofluid is transferred toward the side of the ring-like magnet by centrifugal force. As the operating speed decreases, the magnetic force lines caused by the ring-like magnet retain the ferrofluid thus effecting the magnetic seal mode.
U.S. Pat. No. 4,200,296 (1980, Stahl et al.) discloses a ferrofluid centrifugal seal and method of sealing a rotatable shaft. The ferrofluid is retained in an O-ring seal form at static and low speeds and is moved to a barrier seal in a passageway at high speeds.
FIGS. 1-4 of Japanese Patent Publication Tokukouhei 7-54153 (1995, Koyoseiko) discloses a ferrofluid centrifugal seal, more clearly shown in Prior Art FIG. 2 of the present invention. In this ferrofluid centrifugal seal, an inner side shielding plate is installed between pole pieces from the opposing side. The gap between the housing and the shaft member is made to detour in a U-shaped passageway, as described earlier.
However, the prior art suffers from various disadvantages. The prior art devices do not use conductive ferrofluids, thus are unable to dissipate the increased amounts of static charge developed at higher rotational speeds. The prior art devices cannot be shipped prefilled with ferrofluid because the small volume of ferrofluid and the free-floating nature of the inner shield cause ferrofluid leakage from the U-shaped detour passageway due to the ordinary jostling of the device during transportation. In addition, installing a prefilled prior art device is likely to cause leakage of the ferrofluid because the inner shield is free-floating. An installer needs to attach the inner shield to a shaft. During this process, it is very difficult to prevent the inner shield from moving up against one of the outer poles. This movement causes the ferrofluid to squirt out of the passageway. After installing the apparatus into an actual machine, it is a considerably involved operation to inject any replacement ferrofluid. Further, this makes it difficult to regulate the amount of the ferrofluid within the passageway. Another disadvantage is the fabrication problem of the prior art. In current prior art designs, the seal is shipped to a customer without ferrofluid. Because the inner diameters of the outer poles are relatively small compared to the outer diameter of the inner shield, the space available for injecting ferrofluid into the passageway is rather limited. Thus assembly of the device and injection of the ferrofluid is more difficult and time consuming.
Therefore what is needed is a sealing apparatus that uses a conductive ferrofluid which allows quick dissipation of greater amounts of static charge caused by higher rotational speeds. What is further needed is a sealing apparatus that can be shipped prefilled with ferrofluid without fear of ferrofluid leakage and assembled without ferrofluid squirting out of the sealing apparatus during the installation process. What is still further needed is a sealing apparatus that can operate for longer periods at higher rotational speeds than is currently practical. What is yet still further needed is a sealing apparatus in which the ferrofluid is easier to install. What is further needed is a sealing apparatus that is easier to fabricate.